A few novice wheel questions....

[Anonymous]

"j-p.s" <[email protected]> wrote in message
news:[email protected]...
> On Fri, 07 Mar 2003 21:58:39 +0000, Dave Kahn scrawled: ) The rider's weight does not increase the
> tension in the spokes; it ) only reduces tension in the lower ones.
>
> While doing that it must decrease their extension by a slight displacement of the hub, surely?
> Force as a function of extended length and all that. If the hub is displaced then the upper spokes
> must therefore have an increased extension, the vector sum of all these forces equalling the
> rider's weight.

Spoke is attached at two ends.

It's the bottom of the rim that moves, not the hub. (or to be confusing about it, the hub moves, but
so does most of the rest of the wheel the same amount - the bottom of the dim stays still. Or
somewhere in between!).

cheers, clive

 

[Tim Woodall]

On Sat, 8 Mar 2003 09:42:09 +0000 (UTC), j-p.s <[email protected]> wrote:
> On Fri, 07 Mar 2003 21:58:39 +0000, Dave Kahn scrawled: ) The rider's weight does not increase the
> tension in the spokes; it ) only reduces tension in the lower ones.
>
> While doing that it must decrease their extension by a slight displacement of the hub, surely?
> Force as a function of extended length and all that. If the hub is displaced then the upper spokes
> must therefore have an increased extension, the vector sum of all these forces equalling the
> rider's weight.
>
The rim deforms so although the hub gets closer to the ground, so does the top of the rim by
(almost) the same amount.

Regards,

Tim.

--
God said, "div D = rho, div B = 0, curl E = - @B/@t, curl H = J + @D/@t," and there was light.

http://tjw.hn.org/ http://www.locofungus.btinternet.co.uk/

 

[Ian Smith]

On Sat, 8 Mar 2003, j-p.s <[email protected]> wrote:
> On Fri, 07 Mar 2003 21:58:39 +0000, Dave Kahn scrawled: ) The rider's weight does not increase
> the tension in the spokes; it ) only reduces tension in the lower ones.
>
> While doing that it must decrease their extension by a slight displacement of the hub, surely?
> Force as a function of extended length and all that. If the hub is displaced then the upper
> spokes must therefore have an increased extension,

Nope. You're forgetting that the rim is not rigid. The bottom three or four spokes shorten, but the
other spokes stay much the same length, because the majority of the rim moves downwards by the same
amount as the hub moves downwards, this accomplished by a bending of teh rim in teh vicinity of the
tyre contact patch.

In fact, of the spokes that increase their tension, the ones that do so most are immediately either
side of the contact patch. That is, some of teh lower region spokes _extend_ (though only slightly).

If you are actually interested, there's a fairly detailed FE analysis, with deflected shape plots
and spoke tension plots on my web page at http://www.astounding.org.uk/ian/wheel/ . I did it while
having an argument with a silly merkin. Although it starts off talking about unicycles, teh wheel
analysed is most like a 700C 36 spoke three-cross touring bike type wheel.

The main failing in teh model is that I've just fudged the loading. It could do with an array of
non-linear springs at the bottom to model the tyre behaviour more accurately, but that about
quadruples the complexity of teh model (which is currently a pretty quick linear elastic one with
(at a guess) just a few hundred degrees of freedom).

> the vector sum of all these forces equalling the rider's weight.

Equalling teh applied force, which may be in excess of the riders weight when dynamic effects are
considered.

regards, Ian SMith
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[James Annan]

"Ted" <[email protected]> wrote in message news:<[email protected]>...
> > Ted wrote:
> > > Recently, on my daily ride on my MTB to Wolverhampton, I managed to buckle my rear wheel
> >
> > How? Pothole?
>
> Not a pothole, I just heard a "twang" as I was riding.

If you did this, on an MTB, then I bet you've got **** wheels to start with. In that case, unless
you are desperately short of money it's probably not worth the bother of trying to fix the wheel,
or swapping the rim and keep the spokes, just get a decent pair of wheels, which needn't cost more
than about 100 quid (I think). There's no excuse for decent wheels going badly out of true from a
bumpy road.

James

 

[Huw Pritchard]

On Sat, 08 Mar 2003 01:40:02 +0000, Pete Biggs did issue forth:

> Jobst Brandt (author of The Bicycle Wheel) advises using the existing spokes.

Gerd Schraner (author of The Art of Wheelbuilding) advises using new spokes.

Mind you, I think Schraner works for DT...

--
Huw Pritchard Replace bounce with huw to reply by mail

 

[J-P.S]

On Sat, 8 Mar 2003 11:05:32 +0000 (UTC), Ian Smith scrawled: ) Nope. You're forgetting that the rim
is not rigid. The bottom three ) or four spokes shorten, but the other spokes stay much the same )
length, because the majority of the rim moves downwards by the same ) amount as the hub moves
downwards, this accomplished by a bending of ) teh rim in teh vicinity of the tyre contact patch.

OK. I can see that. Egg-shaped, right? I'm willing to believe the analysis, don't worry.

J-P
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